Broadleaf plants (also written as "broad-leaved") are those with leaves that have a flat, relatively broad surface. This surface is often marked with a network of prominent veins. These botanical characteristics distinguish them from plants with needle-like, awl-like, scale-like, or blade-like leaves. The distinction allows us to group together plants that share the characteristic, for purposes of categorization.
One common use for the word in landscaping and gardening is to refer to garden shrubs and trees that have "regular" leaves, rather than those with leaves shaped like needles, etc. Note that "broadleaf" and "evergreen" aren't necessarily opposites: evergreen plants such as azaleas and rhododendrons, as well as mountain laurels, are broadleaf shrubs, despite sharing the "evergreen" classification with needle-bearing shrubs such as yew. Meanwhile, other evergreen shrubs have awl-like foliage (for example, junipers) or scale-like foliage (for example, arborvitae).
Examples of Trees and Shrubs That Are Broad-Leaved
Some of the following examples are plants with which you have probably been familiar since childhood. They include the deciduous trees whose fall foliage we so eagerly await every autumn:
Broadleaf Plants, Deciduous Plants, and Why Leaves Change Color in Fall
The broadleaf trees and shrubs listed above have developed a strategy whereby they enjoy the best of both worlds. During the warm-weather months, the relatively large surface area of their leaves makes them powerful photosynthesis machines, soaking up as much sunshine as possible. Then, when temperatures fall, they shed their leaves and go dormant. It is a survival mechanism: The large surface area that is such an advantage for the leaves during the warm weather would become a handicap during the cold weather.
This shedding of the leaves is preceded by the glorious fall foliage stage. Why do leaves change color in fall? The tree seals off the leaves from their stems, shutting off their water supply. Thus deprived of water, the leaf ceases to make chlorophyll. It is the chlorophyll that had made the leaves appear green all summer: The chlorophyll was masking other colors in the leaves. So, in a sense, the breathtaking fall-foliage season is the result of an unmasking, in which the leaves' true colors are revealed.
"Deciduous" and "broadleaf" are not, however, synonymous in the world of trees. The live oak (Quercus virginiana) is an example of a broadleaf tree that is evergreen, not deciduous. But this exception should not surprise us, since the live oak is a tree of the American South, where winters are relatively mild.
Why Knowing This Definition Is Useful: Grassy vs. Broadleaf Weeds
"Broadleaf" is not used only to refer to trees and shrubs. The term is also often applied to common lawn weeds fitting that description, to separate them from other weeds for purposes of controlling them through the use of herbicidal sprays. Many beginners fail to realize that how you battle a weed in your lawn very much depends on whether it is broad-leaved or not.
You see, another group of weeds, the so-called "grassy" weeds, are also frequently found in lawns. Because such weeds are botanically similar to the "good" grass (that is, the lawn grass that you wish to keep), you must use special herbicides on them. Otherwise, you would kill your lawn grass in the process.
An example of a grassy weed is crabgrass. In a separate article on this website, you can learn about the best crabgrass killers to use against this villain. An example of a broadleaf weed is clover. You need to use an entirely different type of herbicide to kill clover. When you are at a home improvement store, look for herbicidal products that specifically say (on their packaging) that they are meant for use on broadleaf weeds.
Moving From Botany to Landscape Design
Note that, although broadleaf plants are distinguished from, say, needle-bearing evergreens by the breadth of their leaves, it is hardly the case that all broadleaf plants have particularly large leaves (it does not take much to surpass a skinny pine needle in breadth). For example, boxwood shrubs are broad-leaved, but their leaves are tiny compared to the leaves of big-leaf hydrangeas (Hydrangea macrophylla). Willows are considered broad-leaved yet bear narrow leaves. So although botanists know exactly what they are talking about when they discuss "broadleaf" plants with each other, the term is not as descriptive as a beginner might like it to be.
The difference in leaf size between various broadleaf plants leads to the entirely separate discussion of plant texture. By placing a plant with broader leaves (that is, a coarser texture) next to one with narrower leaves (that is, a finer texture), we can create interesting contrasts in the landscape. This is a matter of landscape design, not of botany.